llvm-cov: Don't manually parse an option for no reason

We're using cl::opt here, but for some reason we're reading out one
particular option by hand instead. This makes -help and the like
behave rather poorly, so let's not do it this way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220928 91177308-0d34-0410-b5e6-96231b3b80d8